ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 



37 



COMPARISON OF HIGH AND LOW TRUE-PRAIRIE AREAS. 



The station maintained in the low-prairie area during 1919 was 

 located in Salt Creek Valley at the foot of the hill, a quarter of a mile 

 distant and 60 feet lower than the one on the hilltop in the high prairie 

 (plate 6). The alluvial soil is Wabash silt-loam. The high water- 

 holding capacity and wilting coefficient (70 and 16.8 per cent for the 

 first foot) indicate the presence of much clay and silt. Indeed, it is 

 very similar to that of the cropped lowland plats at the same level and 

 less than a quarter of a mile distant. A mechanical and chemical ex- 

 amination of the soil from the latter station may be found on page 140. 

 The water-table is reached at a depth of about 9 feet. Using the 

 vegetational cover as the indicator, this area was selected as represent- 

 ing typical low-prairie conditions. A comparison of the water-content 

 at the two prairie stations is given in table 3. 



At the upland station the water-content of the soil was lowered be- 

 yond the wilting coefficient to a depth of 3 feet. At the station in the 

 lowland this condition was reached only once (at 2 feet) below the first 

 foot of soil. However, the greater margin of water-content above the 

 hygroscopic coefficient in the lowland is clearly shown by an examina- 

 tion of the table. The differences are especially marked in the deeper 

 soils. This surplus available moisture has a profound effect upon the 

 structure and development of the vegetation. Undoubtedly during 

 years of drought it is the determining factor in selecting the plant 

 population. The season of 1919 was one of excessive rainfall, espe- 

 cially during the spring and early in summer. 



The evaporating power of the air at the two stations (measured as 

 already described, from May 6 to September 14, 1919) is shown in 

 figure 12. The parallelism of the graphs is a striking feature, but no 



ex. 



AS 

 40 

 35 

 30 

 25 

 £0 

 15 

 10 



Fig. 12. — Graphs showing the average daily evaporation rates in the 

 high prairie (upper line) and low prairie (lower line) at 

 Lincoln, Nebraska, during 1919. 



